BIOLOGICAL NEURON STRUCTUREAND FUNCTIONS.
A neuron, or nerve cell, is an electrically excitable cell that communicates with other cells via specialized
connections called synapses. It is the main component of nervous tissue. Neurons are typically classified
into three types based on their function. Sensory neurons respond to stimuli such as touch, sound, or light
that affect the cells of the sensory organs, and they send signals to the spinal cord or brain. Motor neurons
receive signals from the brain and spinal cord to control everything from muscle contractions to glandular
output. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.
A group of connected neurons is called a neural circuit.
A typical neuron consists of a cell body (soma), dendrites, and a single axon. The soma is usually compact.
The axon and dendrites are filaments that extrude from it. Dendrites typically branch profusely and extend
a few hundred micrometers from the soma. The axon leaves the soma at a swelling called the axon hillock,
and travels for as far as 1 meter in humans or more in other species. It branches but usually maintains a
constant diameter. At the farthest tip of the axon's branches are axon terminals, where the neuron can
transmit a signal across the synapse to another cell. Neurons may lack dendrites or have no axon. The term
neurite is used to describe either a dendrite or an axon, particularly when the cell is undifferentiated.
The soma is the body of the neuron. As it contains the nucleus, most protein synthesis occurs here. The
nucleus can range from 3 to 18 micrometers in diameter.
The dendrites of a neuron are cellular extensions with many branches. This overall shape and structure is
referred to metaphorically as a dendritic tree. This is where the majority of input to the neuron occurs via
the dendritic spine.
The axon is a finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times
the diameter of the soma in length. The axon primarily carries nerve signals away from the soma, and
carries some types of information back to it. Many neurons have only one axon, but this axon may—and
usually will—undergo extensive branching, enabling communication with many target cells. The part of the
axon where it emerges from the soma is called the axon hillock. Besides being an anatomical structure, the
axon hillock also has the greatest density of voltage-dependent sodium channels. This makes it the most
easily excited part of the neuron and the spike initiation zone for the axon. In electrophysiological terms, it
has the most negative threshold potential.
While the axon and axon hillock are generally involved in information outflow, this region can also receive
input from other neurons.
The axon terminal is found at the end of the axon farthest from the soma and contains synapses. Synaptic
boutons are specialized structures where neurotransmitter chemicals are released to communicate with
target neurons. In addition to synaptic boutons at the axon terminal, a neuron may have en passant
boutons, which are located along the length of the axon.
Most neurons receive signals via the dendrites and soma and send out signals down the axon. At the
majority of synapses, signals cross from the axon of one neuron to a dendrite of another. However,
synapses can connect an axon to another axon or a dendrite to another dendrite. The signaling process is
partly electrical and partly chemical. Neurons are electrically excitable, due to maintenance of voltage
gradients across their membranes. If the voltage changes by a large amount over a short interval, the
neuron generates an all-or-nothing electrochemical pulse called an action potential. This potential travels